The present invention relates to a laser driving circuit for driving a laser to emit a laser beam therefrom. In particular, the present invention relates to a recording/reading equipment capable of recording/reading a video signal or the like onto/from an optical disc, such as a CD, a DVD or a Blu-ray Disc (BD).
Generally, in an optical disc, a semiconductor laser element is used for forming a recording mark thereon, and operated to generate a laser beam in a pulsed manner so as to control a heating temperature to prevent the recording mark from being deformed into a tearful eye shape. Late years, in connection with developments aiming for higher-speed recording, a laser-pulse width has become narrower. This involves the need for a laser driving circuit capable of performing a switching action at a higher speed with shorter pulse rise/fall times. Moreover, the higher-speed recording leads to an increase in power consumption of a laser driving circuit, which causes a problem about an increase in heat generation at an optical pickup.
In a conventional laser driving circuit, there has been known a technique of inserting a grounded-base cascode circuit, between a current mirror circuit and a semiconductor laser (i.e., laser diode), as disclosed, for example, in JP 2003-218456 A. This laser driving circuit is intended to suppress impedance fluctuations in the laser and band fluctuations due to the Miller effect so as to achieve shorter rise/fall times in a laser-pulse waveform.
Specifically, in one type of conventional laser driving circuit as shown in FIG. 8, when a base voltage source 808 for a grounded-base cascode transistor 805 is set to have a certain fixed voltage, the base voltage will be determined by a maximum current to be supplied from a driver circuit 801 to a semiconductor laser 804, and an operating voltage which is necessary for the driver circuit 801 to supply the maximum current. Along with a progress in higher-speed recording, the laser 804 is required to have higher laser-beam generating power so as to perform the recording in a shorter period of time, and therefore a current value for driving the laser 804 (i.e., laser driving current) has to be increased. In this case, an influence of an impedance of the driver circuit 801 becomes prominent, causing the need for increasing the operating voltage necessary for the driver circuit 801 and therefore the need for increasing the base voltage of the cascode transistor 805. Moreover, in conjunction with the increase in the base voltage of the cascode transistor 805, it is inevitably required to increase a collector voltage of the cascode transistor 805 and an anode voltage of the laser 804, resulting in increased power consumption.
In the above laser driving circuit, a maximum value of the laser driving current is determined while taking account of production tolerance of the laser 804 and power degradation in an end-of-life stage thereof. That is, the maximum value is set with a relatively large margin with respect to a value during normal use, i.e., the laser driving circuit is operated during the normal use to supply a current lower than a maximum output current thereof (i.e., the maximum laser driving current). Thus, during the normal use, the influence of the impedance of the driver circuit 801 is reduced in proportion to the lower laser driving current, and therefore the operating voltage necessary for the driver circuit 801 becomes lower to allow the base voltage of the cascode transistor 805 to be reduced by just that much. This means that, if the base voltage is fixed based on the maximum output current, wasteful electric power will be consumed during the normal use.
FIG. 9 shows another type of conventional laser driving circuit, wherein a cascode circuit is used in a current discharging type of laser driving circuit. In this laser driving circuit, as a laser driving current is increased, a source voltage of a driver circuit 901 has to be set at a higher value, or a cathode voltage of a semiconductor laser 904 has to be set at a lower value. Further, if each of the source voltage of the driver circuit 901, a base voltage of a grounded-base cascode transistor 905 and the cathode voltage of the laser 904 is determined based on a maximum output current of the driver circuit 901, wasteful electric power will be consumed during normal use, as with the aforementioned type.
FIG. 10 shows an outline of an optical pickup. During an operation of recording/reading a record on an optical disc, a laser driving circuit is operable, according to control of a controller, to supply a current to a semiconductor laser so as to allow the laser to emit a laser beam. This laser beam is emitted onto the optical disc through a collimator lens, a beam splitter and an objective lens. Then, the leaser beam reflected by the optical disc is transmitted to a photodetector through the beam splitter and a cylindrical lens, and converted from an optical signal to an electrical signal through the photodetector. In a recording/reading equipment for an optical disc, the electrical signal is input into the controller through an RF amplifier to read the record. An amount of heat to be generated in the optical pickup is closely related to power consumption in the laser. Thus, a reduction of power consumption in the laser and the laser driving circuit is an effective way to reduce the heat generation in the optical pickup. The reduction of heat generation in the optical pickup is also effective in reducing heat generation in the recording/reading equipment.